Real-estate specific extra filters are installed to municipal water which is already pre-filtered in the water purification plant. They are installed to remove solids and dregs that are separated from the water supply system. The solids and dregs often cause esthetic or taste hindrances, plus over time they accumulate to real estate technology, such as accumulators, washing machines and faucets.
Another common reason are modern impurities which are usually related to emissions caused by manufacturing or other actions from people. Even the modern water purification plants cannot remove all of these impurities. They can be dissolved in water or microscopic particles in a particle shape.